This is a multi-center collaborative study to determine the sensitivity, specificity, positive predictive value, and negative predictive value of contrast enhanced spiral computed tomography (spiral CT) for the diagnosis of acute pulmonary embolism (PE). Pulmonary embolism is common, yet frequently undiagnosed and fatal. Ventilation/perfusion scan is the most frequently used test, but is non-diagnostic in 72 percent of patients with suspected PE, and in 57 percent of patients with proven PE. Pulmonary angiography is the currently definitive diagnostic test, but there is hesitancy to use it because of morbidity,' discomfort, cost, and lack of availability in community hospitals. Contrast enhanced spiral CT is a nearly noninvasive test that offers the possibility of a definitive diagnosis of PE by showing the outline of the thrombus in a pulmonary artery. However, its utility in the diagnosis of PE is unknown. Even though spiral CT has not been adequately validated, many physicians now use it as a definitive and only diagnostic test for PE, or as a definitive test in patients in whom the ventilation/perfusion lung scan is non-diagnostic. This could lead to over treatment or under treatment, and potential serious complications. The role of spiral CT in the diagnosis of PE requires the accurate evaluation that will be made in this investigation. In response to reviewer suggestions on the original application, this revised application proposes to use a composite reference standard based on ventilation / perfusion lung scan, compression ultrasound of the lower extremities, and angiography. The strengths of the proposal are: 1) the importance of the problem of PE in terms of lives lost from under diagnosis, and major bleeding from over diagnosis; 2) the potential applicability of a new technology for the diagnosis of PE that will have widespread availability; and 3) the investigator team.